1. Field of the Invention
The present invention relates to a resin composition having an electromagnetic wave shielding effect, and more particularly to a resin composition for shielding the transmission of electromagnetic waves thereby to prevent disorders or trouble caused by electromagnetic waves.
2. Prior Art
In conventional systems, electronic instruments, such as business machines, electronic computers and television receivers, generate electromagnetic waves by themselves to cause malfunctions and/or noises in neighboring electronic instruments.
On the other hand, the electronic instruments are affected by the electromagnetic waves emitted from the adjacent electronic instruments, leading to malfunction thereof or generation of noises therefrom.
Trouble caused by the electromagnetic waves have been obviated to some extent when the housings of such electronic instruments are made of metal plates or aluminum die castings which can shield the transmission of electromagnetic waves.
However, plastic materials have been predominantly used for the housings of electronic instruments in recent years, because of the merits that they are easily molded to have various designs and that they are light in weight.
However, the plastic materials are generally poor in conductivity and have substantially no electromagnetic wave shielding effect. It is, thus, necessary to process the plastics materials to provide them with electromagnetic wave shielding effect when they are used for the housing of electronic instruments.
Particularly, in recent years, radiation of electromagnetic waves has been severely prohibited by domestic and foreign regulations. Under these circumstances, there is an increasing demand for plastic materials provided with electromagnetic wave shielding effects.
Various methods for providing the plastic materials with the electromagnetic wave shielding effect have hitherto been investigated, the known methods including application of an aluminum foil or a conductive tape, flame spraying with molten zinc, coating with a conductive paint, metal plating on the plastic materials, vacuum evaporation coating, spattering ion plating and molding a conductive plastic material containing a conductive filler.
However, the method of application of an aluminum foil or a conductive tape for the provision of electromagnetic wave shielding effect is not used practically, since it has the disadvantages that extreme skill is required and that it is not suited for housings having complicated shapes.
The method of flame spraying with molten zinc and the method of coating with a conductive paint have been predominantly used at the present time. However, these methods have the disadvantages that the thickness of the lining or coating becomes uneven when the housing has a complicated shape and that the adhesiveness of the lining or coating to the substrate is insufficient, which results in exfoliation of the conductive layer, leading to the loss of the electromagnetic wave shielding effect or even causing a risk of fire.
Although the durability and adhesiveness of the metal plated on the plastics materials are satisfactory, only few kinds of plastic materials can be plated with metals and the articles to be plated are limited to those of small dimensions.
Satisfactory electromagnetic wave shielding effect can be provided by metal evaporation techniques including vacuum evaporation coating, spattering and ion plating. However, these techniques have not been applied for commercial scale production, since they require expensive apparatuses and skillful operations.
Contrary to the aforementioned methods wherein conductive layers are formed on the surfaces of molded plastic materials to provide the electromagnetic wave shielding effect, the molded products made of a composite conductive plastics material containing a conductive filler mixed and dispersed in a matrix plastic material is averted from the impairment of electromagnetic wave shielding effect or from the risk of fire caused by exfoliation of conductive layer.
However, the known conductive plastic molded articles have the disadvantages that satisfactory electromagnetic wave shielding effect cannot be obtained unless a large amount of conductive filler is added to the matrix plastic material, and that the physical properties of the resultant plastic material are deteriorated or the appearance of the molded article is impaired with serious increase in cost as the quantity of the filler added to the matrix plastic material is increased.
Particularly, as the amount of added conductive filler is increased, the dispersibility of the filler is lowered to result in uneven dispersion thereof. Especially when carbon fibers are used for the conductive filler, the fibers are broken during the kneading step to lower the electromagnetic wave shielding effect. If some part of the expensive carbon fibers is replaced by another inexpensive conductive filler in order to decrease the content of the carbon fibers, the fibrous and pulverized fillers present in the mixed condition become hardly dispersed in the matrix resin, leading to deterioration of moldability of the plastic material and deterioration of the properties of the molded articles. If the resultant plastic material is molded at a higher temperature in order to improve the moldability thereof, the matrix resin is decomposed or otherwise damaged so that the physical properties and the appearance of the molded articles are deteriorated and the coloring property of the resin becomes poor.